1. Field of the Invention
The present invention relates to a magnetooptic thin film, a magnetooptic record medium and a method of producing them. More particurarly, the present invention relates to a magnetooptic thin film and a method of producing magnetooptic thin film useful in a record layer of a magnetooptic record medium or the like. Especially, a magnetooptic thin film according to the present invention is provided with a high Kerr rotation angle (.theta..sub.k) in a short wavelength band and a large magnetic anisotropy and can decrease an internal stress even with a thin film thickness range. The present invention is preferably applicable to a record layer or the like for achieving a high record density by using a short wavelength light.
2. Description of the Related Art
In recent years, a magnetooptic record medium has been reduced to practice as a record medium having a high density and a large capacity. Especially, the application field thereof shows a further widening tendency since record information can repeatedly be recorded and reproduced. In accordance therewith, development of a magnetooptic record medium having a higher record density is desired.
According to a current magnetooptic record system, a laser beam having a wavelength of, for example, 830 nm or 780 nm is mainly used. However, a laser beam having a shorter wavelength is effective to realize a magnetooptic record system having a higher record density. The reason is that a beam spot diameter (a diameter of a portion on the surface of a substrate of a medium where a laser beam is condensed) can be decreased in accordance with the shortening of the oscillation wavelength of a laser. As a result, it is known that a pit diameter (minimum record unit of information on a medium) can be downsized and accordingly, the laser beam having a short wavelength is directly connected to promotion of a plane record density. Therefore, it is conceived in a future magnetooptic record system to utilize a laser beam having a short wavelength of 500 through 650 nm that is far shorter than the wavelength currently used. Therefore, a record layer having an excellent magnetooptic characteristic in the short wavelength band is needed as a medium for use in the future magnetooptic record systems.
Currently, for example, a thin film made of a rare earth transitional metal amorphous alloy, represented by a TbFeCo alloy provided with an axis of easy magnetization in a direction vertical to a film face and excellent in the vertical magnetic anisotropy, is frequently used as a record layer material of a magnetooptic record medium on sale. However, the effective magnetooptic characteristics cannot be attained in the above-described short wavelength range by these record layer materials.
As a measure for resolving the problem, an intensive study is being carried out currently in respect of, for example, a structure alternately laminated with thin films made of platinum (Pt) and thin films made of cobalt (Co), that is, a Co/Pt multilayered film or the like. Although such a multilayered film is certainly provided with a Kerr rotation angle to some degree in a short wavelength band, the value ranges from 0.2 to 0.3 degree and therefore, an increase in the Kerr rotation angle has been desired to obtain a high signal to noise ratio (S/N ratio) as the record layer material of the magnetooptic record medium.
Further, in another solution, a Whistler alloy comprising Pt, Mn and Sb (Pt:Mn:Sb=1:1:1 in atomic %) having a crystalline structure of a C.sub.1b formed by a vapor deposition process or a sputtering process is publicly-known (P. G. van Engen et al., Appl. Phys. Lett., 42, 202 (1983)). The PtMnSb whistler alloy film is provided with an excellent characteristic in view of the Kerr rotation angle. However, the PtMnSb whistler alloy film having a crystalline structure of a C.sub.1b type has a drawback where the magnetic anisotropy is small, that is, the saturation magnetization in a direction vertical to the film face is small. This signifies that the alloy cannot be provided with a function necessary as a record layer material of a magnetooptic record medium, that is, an axis of easy magnetization in a direction vertical to the film face (excellent in the vertical magnetic anisotropy). Accordingly, an improvement is needed since recorded information therefore becomes unstable. Meanwhile, a PtMnSb alloy having a crystalline structure and a composition other than those of the Whistler alloy, has almost no magnetooptic characteristic. As described above, the conventional PtMnSb alloy film is insufficient in view of the function as a record layer material of a magnetooptic record medium.
Meanwhile, there has been a report where it has been confirmed that .theta..sub.k &gt;1 degree when a laser beam having a wavelength of 500 nm is irradiated on such a MnSbPt alloy film which has a composition near to Mn.sub.50 Sb.sub.44 Pt.sub.6 (by atomic %) and a crystalline structure which comprises two phases of a C.sub.1b type and a B8.sub.1 type and the film thickness of which is 500 nm (Migaku Takahashi et al., J. Magn. Magn. Mater. 131(1994) 67-75). However, a noble metal of platinum is included in such an alloy film and therefore, it is one of necessary conditions for fabricating a magnetooptic record medium at a low cost to provide an inexpensive medium, to realize a high .theta..sub.k with a thin film thickness. Further, in the case of a sputtered film, stresses are generally liable to cause, with the film thickness of 50 nm or less, the adherence thereof with a substrate or a protective film which is unstable, and which is devoid of long period reliability, that is not preferable. Accordingly, it has been desired to establish film forming conditions capable of realizing a high .theta..sub.k with a thin film thickness and where internal stresses are small.
It is an object of present invention to provide a magnetooptic thin film which is comprised by a film thickness of less than 500 nm, which has a Kerr rotation angle (.theta..sub.k) in a short wavelength band of 650 nm or less, a large saturation magnetization in a direction orthogonal to the film face and small internal stresses, a magnetooptic record medium and a method of producing them.